Optical imaging systems typically include a transmissive or a reflective imaging panel, also referred to as a light valve or light valve array, which imposes an image on a light beam. Transmissive imaging panels are typically translucent and allow light to pass through. Reflective imaging panels, on the other hand, reflect only selected portions of the input beam to form an image. Reflective imaging panels, such as liquid-crystal-on-silicon (LCoS) microdisplays, provide some advantages, as controlling circuitry may be placed behind the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness.
LCoS-based imaging panels, as well as other type of liquid crystal display (LCD) based imaging panels, rotate the polarization of incident light. Typically, polarized light is either reflected by the imaging panel with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired grey scale. A 90° rotation provides the brightest state in these systems. Accordingly, a polarized light beam is generally used as the input beam for reflective LCD imagers.
Projection display systems based on the LCD technology commonly employ one, two, or three LCD imaging panels in order to create a full color projected image. When three panels are used, each panel is illuminated with one of the three primary colors. Each panel is electronically addressed with video data corresponding to the color channel for the illuminating light received at that imager. Finally these three monochrome images are projected onto the screen simultaneously, resulting in a high brightness image that has no artifacts associated with temporal color sequencing of the image data.
In one panel systems, such as those disclosed in U.S. Pat. No. 6,702,446 issued Mar. 9, 2004 to De Vaan et al, and U.S. Pat. No. 6,707,516 issued Mar. 16, 2004 to Johnson et al, some means of illuminating the single imager sequentially with all three primary colors is employed, typically either color-sequencing or scrolling a pattern of three primary-colored stripes of light across the imager, e.g. using a color wheel, to create a full color image. The imager is electronically addressed with a time-sequential (and possibly scrolling) video image data stream that modulates in synchrony with the time-(and possibly space-) varying multi-color illumination source.
An imaging system projects a magnified image of this color-sequential (or color scrolling) picture onto a viewing surface where the viewer perceives a full-color image as a result of the human eye's slower response time compared with the rate of color modulation in the imaging system. Due to the color-sequential nature of the one-panel display, a lower-brightness image results as compared with three-panel displays. The image may also include color-breakup artifacts due to the temporal nature of the color sequencing system.
A two-panel architecture is an attractive compromise between these two extremes. Typically, color management in a two-panel architecture falls in one of several generals schemes. In a first scheme, light from the illumination source is divided into two beams by splitting the raw light into its two constituent, orthogonal polarization states, forming to continuous, i.e. non-modulated, polarized beams. Each of these beams is routed to one of the two imagers. These two polarized beams are color filtered and modulated with color sequencing or color scrolling means, e.g. a color wheel, similar to systems having only a single imager.
The two resulting color-modulated images are recombined using a polarization beam combiner to create a single color-sequential full-color image. The benefit of this approach compared to a one-panel system is a brighter image than that obtained using only one imager. However this system is still not as bright as a three-panel system since some form of temporal color sequencing is still needed.
In a second scheme disclosed in U.S. Pat. No. 6,280,034 issued Aug. 28, 2001 to Brennesholtz, and U.S. Pat. No. 6,388,718 issued May 14, 2002 to Yoo et al, light from the source is first spectrally divided into two beams such that one of the beams consists of light from a single primary color channel (for instance only red light) and the second beam consists of light from the remaining two primary color channels (green and blue light, for example). Light from the first beam is routed to one of the panels so that this panel continuously receives one primary illumination color and displays image data corresponding to this one primary color. The second beam, consisting of light from the two remaining primary colors, is directed to the second imaging panel. Dynamic color sequencing or color scrolling means are used to temporally sequence the two primary colors of the second beam onto the imaging panel.
The imaging panel is electronically addressed with a time-sequential video image data stream that modulates in synchrony with the time-varying (and possibly space-varying, e.g. scrolling) two-color illumination source.
The images from the two imaging panels are optically combined using a dichroic beam-combining element and are projected onto a screen or viewing surface to create a full-color image. This system may optionally include a polarization recovery subsystem in the illumination subsystem to increase overall display brightness. Nevertheless, the resulting image is less bright than a full three-panel system due to the temporal color sequencing in the two-color imager. However, it is typically brighter than a one panel system because it is capable of simultaneously projecting two overlapping, full-frame color images, whereas the single panel system only displays one full-frame color image at any instant in time.
All of these prior-art two-panel projection systems require dynamic color-separating means, typically either mechanical or electrical color shutters, to provide the color sequencing upon one of the imaging panels. Among them, mechanical color wheels are currently most widely used. For example, U.S. Pat. No. 5,517,340 issued May 14, 1996 to Doany et al, and U.S. Pat. No. 5,863,125 issued Jan. 26, 1999 to Doany disclose two-panel schemes in which color wheels are used to sequentially provide one of the primary colors to a polarization beam splitting cube for projection. The color wheels as means of dynamic color separation have however some disadvantages, such as a fixed color ratio that complicates white balance adjustment, relatively large size and cost, and mechanical wear.
U.S. Pat. No. 6,568,815 issued May 27, 2003 to Yano et al, and U.S. Pat. No. 6,650,377 issued Nov. 18, 2003 to Robinson et al disclose dual-panel systems which use color-selective dynamic polarization control of the illuminating light beam for color sequencing. These two-panel projection systems include an input color-selective polarization modulator, or switch, to first dynamically rotate the polarization of one color component differently than the polarization of a second color component of the beam, and then use polarization beam splitters to control which primary and or secondary colors are provided to the panels. These dynamic color-selective polarization switches, which are embodied as a retardance-based polarization stack filter that comprises a stack of birefringent layers and at least one active liquid crystal cell, also have certain disadvantages; they don't have sharp transitions from one color-band to the next, and may be difficult to fabricate using standard technological processes, and can therefore be relatively expensive.
An object of the present invention is to overcome the shortcomings of the prior art by providing a two-panel LCD system, which employs dynamic achromatic polarization switching and static color separation for providing temporal color sequencing.
Another object of the present invention is to provide a two-panel LCD system using a simple input LC polarization switch and static color separators, in which both of the imaging panels are time-shared between two colors.